Donald F. Boesch, Donald A. Anderson, Rita A. Horner, Sandra
E. Shumway,Patricia A. Tester and Terry E. Whitledge. 1997. Harmful
Algal Blooms in Coastal Waters: Options for Prevention, Control
and Mitigation. NOAA Coastal Ocean Program, Decision Analysis
Series No. 10, Special Joint Report with the National Fish and
Wildlife Foundation, February 1997.
Paralytic Shellfish Poisoning
Paralytic shellfish poisoning (PSP) is a significant problem
on both the east and west coasts of the U.S. Caused by several
closely related species in the genus Alexandrium,
PSP toxins are responsible for persistent problems due to their
accumulation in filter feeding shellfish (e.g., Shumway et al.
1988), but they also move through the food chain, affecting zooplankton,
fish larvae, adult fish, and even birds and marine mammals (Anderson
and White 1992; Geraci et al. 1989; Shumway 1995). On the east
coast, PSP is a serious and recurrent problem from Maine to Massachusetts.
Connecticut, Long Island (New York) and New Jersey occasionally
experience the toxin (or Alexandrium) at low levels, but these
areas seem to define the southern extreme of this organism's geographic
distribution. The offshore waters of George's Bank experienced
a serious PSP outbreak several years ago, leading to the extended
closure of the surfclam fisheries and the demise of a fledgling
roe-on scallop fishery. On the west coast, PSP is a recurrent
annual problem along the coasts of northern California, Oregon,
Washington, and Alaska. Overall, PSP affects more coastline than
any other HAB problem.
It is likely that seasonally recurring outbreaks of PSP are
linked to the existence of a dormant cyst stage in the Alexandrium
life history. This strategy allows the species to deposit dormant
cells in sediments where they survive through harsh winter conditions
and then germinate to initiate new outbreaks in subsequent years.
Prior to 1972, for example, PSP was restricted to the far eastern
sections of Maine ("down east") near the Canadian border.
That year, however, a massive red tide causing high levels of
toxicity in those areas for the first time recorded in history.
Virtually every year since that event, this region has experienced
PSP outbreaks, a result of the successful colonization of the
area by Alexandrium spp. A similar expansion, with subsequent
recurring outbreaks of Alexandrium, occurred in the Puget Sound
region of Washington in the late 1970's an area with no prior
history of shellfish poisoning (Nishitani and Chew 1988). Long-term
climatic variability, which affects temperature, upwelling, and
currents or allows cysts to survive in areas where they did not
before, may be factors in such range extensions.
PSP occurs over a large geographic range, so a variety of physical
mechanisms underlie the spreading of Alexandrium blooms. In southern
New England , for example, localized blooms occur in small, isolated
salt ponds and embayments, whereas in the southwestern Gulf of
Maine, linkage has been documented between the abundance and distribution
of Alexandrium and a buoyant coastal current that travels from
north to south in that region (Franks and Anderson, 1992). Fresh
water enters the Gulf of Maine from several large rivers in southern
Maine, and the freshened coastal waters flow south in a manner
that is influenced by the amount of rainfall and snowmelt, the
local wind stress, and the underlying circulation of the Gulf
of Maine. Toxic Alexandrium populations are closely associated
with this buoyant water mass. The long distance transport of Alexandrium
cells in this coastal current are responsible for PSP outbreaks
in southern Maine and Massachusetts, and may even be linked to
shellfish toxicity on George's Bank (Anderson and Keafer, 1992).
The hydrographic mechanisms underlying PSP blooms in down-east
Maine are more poorly understood than those described for the
region to the southwest.
Similarly, on the west, blooms can be either localized in distribution
(i.e., restricted to the inland waters of Puget Sound or the fjords
of Alaska) or wide spread along the Pacific Ocean coast. In northern
California, it is hypothesized that the onset of PSP toxicity
is linked to the onshore movement of warm, stratified waters following
the relaxation of coastal upwelling (Horner et al. in press).
The relaxation events or downwelling, brought about by a change
in wind speed or direction, carry established Alexandrium populations
toward shore, resulting in rapid increases in toxicity in nearshore
shellfish. There is currently no evidence that this also occurs
in Washington or Alaska.
These are but a few of the physical mechanisms underlying PSP
outbreaks in the U.S. Some areas are well-studied, and others
are virtually unknown. Alexandrium blooms generally do not involve
large cell accumulations that discolor the water and may be below
the water surface where they are not visible. Low density populations
can cause severe problems due to the high potency of the toxins
produced by these species. Furthermore, Alexandrium species can
grow in relatively pristine waters, and it is difficult to argue
that anthropogenic nutrient inputs are stimulating the blooms.
These characteristics are important when considering mitigation
and control strategies.
The economic impact of these outbreaks is significant, though
difficult to estimate in total. Most of the states listed above
operate shellfish monitoring programs, each of which costs $100,00-200,000
per year. Estimates of the losses to shellfisheries and other
seafood-related industries are few, but one listed the costs of
a single PSP outbreak in Maine at $6 million (Shumway et al. 1988).
Some estimates place the value of the quarantined surfclam resources
on George's Bank at several million dollars per year. This resource
has been closed to harvest since 1989. On the west coast, the
shellfish industry in Alaska, which produced 5 million pounds
of product in 1917, has been greatly reduced (except for aquaculture)
as a direct result of persistent product contamination of butterclams
by PSP (Neve and Reichart 1984). There is a highly restricted
recreational shellfish industry since many if the state's resources
remain permanently closed due to high costs associated with monitoring
the state's vast coastline. The value of the sustainable, but
presently unexploited, shellfish resource in Alaska is estimated
to be $50 million per year (Neve and Reichart 1984). In addition
to the risks of PSP from molluscs, there are PSP and domoic acid
poisoning risks from consumption of Dungeness and other crabs.
To report problems or provide comments,
Andrew Kane (Aquatic Pathobiology Center) at:email@example.com
Dan Jacobs (Maryland Sea Grant)
UNIVERSITY OF MARYLAND
College of Ag & Natural Resources
Department of Veterinary Medicine
Aquatic Pathobiology Center